I added this question as an aside in another topic and seemed to have killed that topic, so let me try it on its own.

If a spirit /vessel duo are stunned to unconsciousness, how does the spirit resist an ensuing banishment?

a) Does it not resist?

b) Does it resist with some modifier?

c) does it resist as normal?

d) Does some other option happen?

I'd not given a lot of thought to possessed vessels being knocked out.

I think if the vessel is rendered unconscious the possessing spirit is disrupted and returns to the metaplanes. But I've not got my book on me, so I could be wrong.

There is no Spirit/Vessel there is only one Dual Natured thingie. If it is knocked unconscious then the possessing spirit is disrupted to its Home MetaPlane.

A spirit posessing a living being is only disrupted if the physical damage track is filled (p94 SM).
While being knocked out I'd still allow a possession spirit to resist being banished, just like I'd allow a knocked out character a body roll to resist poison. I would apply the appropriate wound modifiers though.

I hope you mean on the spirit to resist banishing, and not on the character who is poisoned to resist the poison.

Absotively.

then of course, if the spirit is unconscious it doesn't really represent a threat and I'd probably ignore it and move on.
I'm pretty sure from the bbb that if a spirit fills it's stun damage it's disrupted.

If it's your friend thats possessed you'd want to get rid of the spirit whether it's a threat or not. Knocking it out is a decent way to make sure it holds still while you try to banish it.

Materializing Spirits get disrupted whenever the stun track is full. Possession spirits only get disrupted if they in the astral and get hit with enough stun to fill their stun track.

So it would appear the consensus is that banishment proceeds as normal.

I guess if a spirit possesses a host that is incapacitated, it may just evacuate the host rather than submit to banishment or disruption. However, the instructions from the summoner may demand the spirit stay in place, for example to continue to provide its immunity to normal weapons to the host.

The spirit does not have a separate Stun track from his host while possessed and takes all the damage with him in case he somehow manages to leave the host while unconscious. That would mean instantaneous disruption.